Significant amounts of aluminum may be recovered from clays and other low grade aluminum-bearing ores by acid leaching of the ore (for example, a clay which preferably has been calcined) to yield a solution containing soluble aluminum Al.sup.+ .sup.+.sup.+ as the sulfate, the nitrate or the chloride. Such solution normally may also contain undesirable ions, and, in particular, iron as Fe.sup.+.sup.+.sup.+ . Iron is most undesirable since its presence interferes with the economic production of high purity aluminum metal.
Prior art processes, typified by that disclosed in U.S. Pat. No. 3,211,521, teach the purification of liquids containing aluminum and iron ions by the process wherein an aqueous solution thereof is contacted with an alkyl-substituted phosphoric acid, HRR'PO.sub.4 wherein R is a straight chain or branched chain aklyl radical containing at least 8 carbon atoms and R' is a hydrogen or alkyl. Typical of such compositions is dioctyl phosphoric acid. In use of such techniques, the alkyl-substituted phosphoric acid is contacted with aqueous liquor containing aluminum and iron; and the alkyl-substituted phosphoric acid will become loaded with iron without picking up any appreciable quantity of aluminum. The result is the aqueous liquor must still be further treated to separate the aluminum from the other undesired impurities. The result is extended processing which is costly in terms of time and capital equipment.
Efforts to use aryl-substitued phosphates are typified by U.S. Pat. No. 3,320,032 in which benzyl, naphthyl, and tolyl radicals are disclosed. However, none of these are selective for aluminum and, in fact, the entire disclosure of this patent is to use organic hydrogen phosphates to selectively extract iron from aluminum solutions; not to remove aluminum selectively from an impure solution.